This is definitely an abstract concept for students seeing as they only think of a circle as a round object with zero sides (seriously what they think). Any who...I created this PowerPoint Presentation last year to help my children understand the parts of a circle. I also have a song to help them remember the parts of a circle. Of all my songs, it is my favorite. It is to the tune of Right Round by Flo' Rida.
Check out the presentation at my TpT store. There is a little preview below:
The presentation includes interactive activities, notes for students, an art project, real life examples, a song about circles, and test prep. One activity I do on the second day of my unit on circles, is to go out side and create a circle diagram in the parking lot using sidewalk chalk. We form a huge circle including the whole class and each find the center, walk the radius and diameter, and find how many "feet" the circumference measures. Not only do we have fun, but students, parents, and teachers learn from our artwork as they walk or drive through the parking lot.
Below is a couple of the videos I show while teaching this unit.